1. Field of the Invention
The present invention relates to a bipolar transistor and its manufacturing method.
2. Description of the Related Art
A lateral-structure pnp bipolar transistor is widely used in the art of manufacturing bipolar transistor integrated circuits, since it can be made simultaneously by use of the same manufacturing process as an npn bipolar transistor. As is well known, however, the current amplification factor and frequency characteristic of the lateral-structure pnp bipolar transistor are not very satisfactory. In recent years, a large number of small-sized, high-speed operation self-alignment type npn bipolar transistors have come to be manufactured, wherein the layer for leading out the base electrode is formed of polysilicon. In accordance with this situation, a large number of self-alignment type lateral-structure pnp bipolar transistors have come to be manufactured since they can be formed simultaneously by use of the same process as the self-alignment type npn bipolar transistors.
The self-alignment type lateral-structure pnp bipolar transistor will be explained, with reference to its sectional view shown FIG. 1.
In FIG. 1, reference numeral 401 denotes a p-type semiconductor substrate; 402 denotes a high-concentration n.sup.+ -type region; 403 denotes a high-concentration p.sup.+ -type channel cut region; 404 denotes an n-type epitaxial layer; 405 and 406 denote high-concentration p.sup.+ -type regions, respectively; 407 denotes an element-isolation region; 408, 409 and 410 denote polysilicon layers, respectively; and 413, 414 and 415 denote electrodes formed of aluminum, respectively. The n-type regions 402 and 404 jointly constitute a base region, the high-concentration p.sup.+ -type region indicated with 405 constitutes a collector region, and the high-concentration p.sup.+ -type region indicated with 406 constitutes an emitter region. The polysilicon layer indicated with 408 serves as a collector-leading layer for leading out the collector the polysilicon layer indicated with 409 serves as a base-leading layer for leading out the base, and the polysilicon layer indicated with 410 serves as an emitter-leading layer for leading out the emitter. The aluminum electrode indicated with 413 serves as a collector electrode, the aluminum electrode indicated with 414 serves as an emitter electrode, and the aluminum electrode indicated with 415 serves as a base electrode.
In the semiconductor device of the above structure, the high-concentration p.sup.+ -type collector region 405 is formed by diffusing impurities from the polysilicon layer 408 (i.e., the collector-leading layer). Likewise, the high-concentration p.sup.+ -type emitter region 406 is formed by diffusing impurities from the polysilicon layer 410 (i.e., the emitter-leading layer). However, since the polysilicon layers 408 and 410 are patterned by use of the lithography technique using photoresist, the base width is determined almost exclusively by the image resolution limit W of the lithography technique, i.e., the minimum design rule.
In reality, the presence of the high-concentration p.sup.+ -type diffusion regions 404 and 405 makes the base width slightly smaller than the base width determined by the image resolution limit W. Therefore, the base width of a self-alignment type lateral-structure bipolar transistor can be smaller than that of a lateral-structure bipolar transistor which is not of a self-alignment type, so that the frequency characteristics of the former can be more improved than those of the latter. Under the circumstances, it may be thought to diffuse a large amount of impurities from the polysilicon layers 408 and 410 in the lateral direction, so as to reduce the base width further. However, even if a large amount of impurities are diffused, the base width can be reduced only in the surface regions of the substrate; it remains almost unchanged in the regions which are deep from the surface of the substrate. This is because impurities diffuse radially and do not diffuse in the lateral direction alone. Thus, even if impurities are diffused in large quantities, the base width cannot be greatly reduced, and the frequency characteristics cannot be improved, as intended. In summary, it is safe to say that in actual devices, the image resolution limit W of the lithography technique exclusively determines the base width. Since the image resolution cannot be easily improved by use of the presently-available technology, the base width has to be reduced by using some other method.
As mentioned above, impurities cannot be diffused in the lateral direction alone. They are inevitably diffused in the vertical direction as well. Naturally, therefore, both the collector region 405 and the emitter region 406 become deeper as a result of the impurity diffusion. If they are deepened, the emitter junction which opposes the collector junction will increase in size, resulting in improvement of the current amplification factor. However, the increase in the size of the emitter junction prevents the fabrication of a small-sized element.
In the above self-alignment type lateral-structure bipolar transistor and its manufacturing method, the base width is dependent largely on the image resolution limit W of the lithography technique. For this reason, it is impossible to obtain a lateral-structure pnp bipolar transistor which has improved frequency characteristics and is suitable for the reduction of the size of an element.